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Canadian Journal of Physics

Numerical analysis of short laser pulse superposition in a fluorescent dye medium

D. Faubert, S. L. Chin, M. Cormier, M. Boloten

Published Feb 1, 1979• 5 citations
Publisher

Abstract

We present a rate equation analysis of passive short laser pulse superposition in a fluorescent dye medium, where it is seen that a single dye solution may act as both an absorber and an amplifier. Amplification proceeds via stimulated emission following a population inversion created with the energy the dye gained through absorption. Numerical results are compared with published experimental data. The agreement between the two is qualitative.

DOI
10.1139/p79-021

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IEEE Journal of Quantum Electronics•Sep 1, 1979•10 citations
We present a rate equation analysis of passive short laser pulse superposition in a fluorescent dye medium, where it is seen that a single dye solution may act as both an absorber and an amplifier. Amplification proceeds via stimulated emission following a population inversion created with the energy the dye gained through absorption. Numerical results are compared with published experimental data. The agreement between the two is qualitative.
We have developed a self-consistent description, based on the semiclassical radiation theory of the propagation of a light pulse in a dye medium. The multilevel nature of the energy bands of a dye molecule and the solvent cage effect in a solution are taken into account. In the case of light pulses whose duration is much shorter than the solvent-solute relaxation time but much longer than the time of atomic phase memory, the latter being of the order of ${10}^{\ensuremath{-}12}$ sec for dyes, we solve the coupled system of equations for the field and the medium by the perturbation procedure. The hole-burning effect is incorporated into the solution in a natural manner. This is especially clear in the direct transition approximation where all the intermediate transitions are neglected. A recent idea of pulse superposition, consisting in storing the energy of a pulse in the form of population inversion followed by stimulated emission triggered by another pulse, is further discussed. Contrary to the orginal model developed by one of us (S.L.C.), we show that no cw pumping is required. The feasibility of the pulse superposition is supported by some experimental results obtained by sending a train of 10-psec pulses through a glycerine solution of oxazine 1 perchlorate.
A report is given of a study of the amplitude-spectral asymmetry of the stimulated emission of superfluorescence from dye solutions pumped longitudinally by nanosecond laser pulses. This amplitude asymmetry is manifested by the difference between the powers of the forward and backward waves, and it is a function of the rate of pumping and of the optical density of the active medium. The asymmetry of stimulated emission from a ring laser is manifested by the appearance of a wave with a traveling-wave ratio of ~90 in the absence of any optical isolators. The spectral asymmetry of a ring laser is manifested by a difference between the spectra of strong and weak waves traveling in opposite directions. A quantitative analysis is made of the effects based on the solution of the transfer equations for radiation traveling in a nonlinearly absorbing active medium.
We report an observation of nonsaturable absorption of light. When a laser medium in a small aspect ratio geometry is uniformly and transversely pumped, amplified spontaneous emission (ASE) builds up along the longitudinal axis. The higher ASE intensity at the ends of the cell repopulates the ground state through the stimulated emission process more rapidly than at the center of the cell. Therefore, the absorption of the pump light is enhanced at the ends and the transmitted pump beam shows a nonuniform profile with a peak at the center. When the occupation of the states other than the upper lasing state and the ground state can be ignored, the normalized profile of the transmitted pump beam becomes identical to that of the fluorescence depicting the upper state population distribution. This fact has been confirmed experimentally with a R6G dye cell pumped with various intensities of frequency doubled Nd:YAG laser (λ = 532 nm, 10-ns pulse duration). The percentage of the absorbed pump energy at the ends of the cell remains the same, virtually nonsaturable, whereas at the center it decreases as the pump intensity increases.
Equations are proposed for the nonlinear interaction between optical pulses and dye solutions allowing for the contribution of the stimulated emission band to the nonlinear dispersion of the solution. A numerical solution of the equations showed that, for pulses having a carrier frequency near the absorption band of the solution, phase modulation of the pulses transmitted by the solution took the form of a deviation of the instantaneous frequency toward the red, and the "chirp" was positive.
Amplified spontaneous emission of laser media having a spherical or disk shape is numerically analyzed. Some properties are discussed and a comparison with linear systems is made. Analytical approximations, for the case of high pumping rates and high efficiency, are obtained. Energy level schemes of dye systems are used in the analysis and in the numerical calculations. A coupled set of radiative transfer and rate equations is solved under quasi-steady-state conditions. The model takes into account the fluorescence spectrum and gain saturation effects with radially symmetric distributions of the active dye molecular densities. As a comparison, the numerical examples also include an H2O system which is relevant in astrophysics.